Xycraft is crashing when it attempts world generation. You either have a corrupted save, or this is a Xycraft bug. The simplest solution would be to deactivate Xycraft.
You could also check if this error occurs with a new world. If it does, re-install Xycraft. I doubt this will happen though.
Another "solution" I can think of is to edit the player.dat file to reposition yourself and not venture out far enough to trigger world generation.
You could also try updating Xycraft, it may be that this issue is fixed in newer versions.